A technique for measuring a three-dimensional position of a work machine by using the GPS (Global Positioning System) and the like, using the obtained position information about the work machine to manage the work machine, manage a construction status performed by the work machine, and control the work machine has been known. Patent Literature 1 describes a technique for accurately measuring the position of a monitor point even when there is a change in the measurement precision of the three-dimensional position of a work machine.